How to fix ‘Notice: Undefined index:’ in PHP form action

Assuming you only copy/pasted the relevant code and your form includes <form method="POST">

---

if(isset($_POST['filename'])){
    $filename = $_POST['filename'];
}
if(isset($filename)){ 
    echo $filename;
}

If _POST is not set the filename variable won’t be either in the above example.

An alternative way:

$filename = false;
if(isset($_POST['filename'])){
    $filename = $_POST['filename'];
 } 
    echo $filename; //guarenteed to be set so isset not needed

In this example filename is set regardless of the situation with _POST. This should demonstrate the use of isset nicely.

More information here: http://php.net/manual/en/function.isset.php